Why are invasive species bad?

Taken from: http://www.sgnis.org/publicat/slide/pl_s4.htmlInvasive
species have large impacts on the ecosystems they invade. They can
affect the chemical makeup of the water, availability of resources, and
even alter the food web. Invasive species impact other species more
directly through competition for resources. Because invasive species
evolved with other competitors and native species evolved in the
absence of invasive species they lack the ability to compete with
invasive species. This gives invasive species the upper hand when
resources become scarce. This leads to an overall decrease in
biodiversity as the area invaded becomes less populated by native
species and more taken over by invasive species.

Invasive species are also expensive.
They are costly to society in many ways through both, direct
management, and decreased production. The annual estimated cost in the
United States of invasive species control is about 120 billion dollars
per year. It is estimated that the global cost of invasive species
management is equal to five percent of the global economy.